Typically, switched mode DC-DC converters that are operable using a battery, use different modes of operation for ensuring high efficiency over a wide range of output loads. More specifically, the operation of the switched mode DC-DC converter in different modes ensures longer battery charge utilization. For example, at high load currents, fixed frequency Pulse Width Modulation (PWM) mode may provide higher efficiency compared to Pulse Frequency Modulation (PFM) mode. Similarly, at lower loads, PFM mode may provide comparable efficiency when operating at low switching frequencies. As a result, it may be required to switch between different modes in order to get the best efficiency across the whole range of output load current.
As known, there are several schemes for automatic transitioning between PFM and PWM modes of operation in a switching regulator. Typically, the load current is compared against a threshold in order to initiate the transition between PFM and PWM modes.
In one example, when the switching regulator is operating in PFM mode, the load current may be estimated using one of output voltage ripple and peak inductor current. However, the methods discussed above are highly inaccurate as they are dependent on various factors such as supply voltage, silicon process corners, temperature and tolerance of passive components used with the switching regulator.
In another example, the load current in PFM mode is estimated using the switching frequency. However, the load current estimated using the switching frequency may be inaccurate as the switching frequency is dependent on parameters such as supply voltage and temperature. Further, it is difficult to quantify the switching frequency as a function of the load current. Consequently, setting of the threshold for a given load is based on simulation rather than calculation.
In yet another example, the load current in PFM mode may be estimated using a sense resistor in the PMOS path at the output of the switching regulator. However, the conversion of sensed current to equivalent load current may not be easy. Further, the conversion of sensed current may also lead to inaccuracies in the estimated load current. Further, the sense resistor may result in additional losses in the PMOS path.
In yet another example, load current of a switching regulator operating in PWM mode is estimated based on a dead-time. However, as the dead-time depends on factors such as supply voltage and silicon process corners, the load current estimated may be inaccurate.
Therefore, there is a need in the art to accurately transition between PFM and PWM modes.